Holography offers much promise as a storage medium, especially for images, and as pattern recognizers. See, for example, U.S. Pat. No. 4,988,153 issued Jan. 29, 1991 and U.S. patent applications Ser. No. 07/588,710, filed Sep. 27, 1990, now U.S. Pat. No. 5,121,228, issued Jun. 9, 1992 and Ser. No. 07/713,421, filed Jun. 10, 1991 now U.S. Pat. No. 5,138,489, issued Aug. 11, 1992, all by Paek. These holographic devices share a common configuration, as illustrated schematically in FIG. 1, for the recording and readout of the hologram. During the recording phase, a holographic recording medium 10 is simultaneously irradiated by an image beam 12 and a reference beam 14 which are coherent with each other and are angularly separated by the angle .theta..sub.R, with the image beam 12 being displaced from the surface normal by the angle .phi.. The image beam 12 in some sense bears an image, but its exact nature depends on the nature of the device being implemented. The holographic recording medium 10 is photo-sensitive to the light of the two beams 12 and 14, and the interference between them causes a hologram to be recorded in the recording medium 10. The hologram may be considered as a series of diffraction gratings recorded in the medium 10 corresponding to the Fourier transforms of the image in the image beam 12. Multiple holographic images can be recorded in the recording medium 10 by moving the reference beam 14 to a slightly different angular position .theta. near .theta..sub.R.
If the device is used as an image pattern recognizer, during the read-out, an unknown image (assumed at this point to exactly correspond to one of the recorded images) is impressed on the image beam 12 and irradiates the recorded medium 10. No reference beam is used. The unknown image interacts with all the recorded holograms and is diffracted into a single beam 16 at an angle .theta..sub.1 with respect to the axis of the image beam 12; the angle .theta..sub.1 is close to .theta..sub.R. That is, the diffracted beam 16 is coelinear to the reference beam 14 used to record that corresponding image. Accordingly, by determining the angle .theta..sub.1 of the diffracted beam 16, the image or pattern is recognized. Such a pattern recognizer becomes powerful when queried with images that closely but not exactly resemble one of the recorded images. In this case, the diffracted beam 16 at the one angle .theta..sub.1 having the greatest intensity indicates the recorded image most closely resembling the unknown image.
Holographic devices are typically limited by the photosensitivity of the recording medium 10. An example of such recording media for planar holograms is a thermoplastic plate, which is a thin plastic plate abutted to a photoconductor and which deforms in accordance with variations in light exposure. Such thermoplastic plates are described by Collier et al. in the text Optical Holography (Academic Press, 1971), pp. 298-305 and in Operator's Manual: HC-300 Holographic Recording Device from the Newport Corporation. As is well known, the diffraction efficiency for thermoplastic plates varies with the recording angle .theta..sub.R. The sensitivity of thermoplastics is limited to a fairly narrow bandwidth of spatial frequency centered at about 800 lines per millimeter although the value may vary between 600 and 1200 lines per millimeter for different thermoplastics. A typical response curve 20 as a function of angle recording angle .theta..sub.R for 514.5 nm light is illustrated in FIG. 2 and has a peak 22 with a relatively narrow bandwidth at about .theta..sub.P =20.degree.. The peak efficiency is about 12% for thermoplastics. Once the peak recording angle .theta..sub.P has been determined, in conventional practice, that angle .theta..sub.P is used as the central recording angle .theta..sub.R and any of the recording angles .theta. must be close to the peak 22. That is, to maximize intensity, the recording geometry is usually chosen so that .theta..sub.R .apprxeq..theta..sub.P.
The diffracted beam 16 in fact represents the first-order diffraction beam at the first-order diffraction angle .theta..sub.1 =.theta..sub.R. Additional, higher-order beams are diffracted, for example, a second-order diffraction beam 18 at the angle .theta..sub.2 from the axis of the image beam 12. In general, the angle for an n-th order diffraction beam is given by ##EQU1## where .lambda. is the wavelength of light and d is the period of the diffraction grating recorded in the hologram. If the reference beam 14 is normal to the surface so that .phi.=0, the equation simplifies to ##EQU2## If the angles .theta..sub.n of interest are small, then EQU .theta..sub.n =n.theta..sub.1, (3)
where .theta..sub.1 is the position of the first-order diffraction beam.
There are many applications where the higher-order diffraction beam would be useful. However, as shown in FIG. 2, the diffraction efficiency would be substantially reduced for the higher-order beams, for example, to about 1% for thermoplastics.